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I read a paper titled "Martian magmas contain abundant chlorine, but little water" in which it's mentioned that Mars could have up to 2.5 times more chlorine than Earth. Considering that Mars and Earth were created very close and at the same time, this is surprising.

This made me think about the mechanisms that could deposit large amounts of chlorine on a planet, so I tried to find information about it, but I did not find anything about it on the web.

So, how could a planet have large amounts of chlorine even though chlorine is much less abundant than other elements?

Even though chlorine is still not abundant, I think there might be enough for a chlorine planet to be created... only if enough chlorine can accumulate in a specific region of the protoplanetary disk.

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    $\begingroup$ Just a guess here, I don't have any evidence backing this up. All materials in the proto planetary disc have what's called a Frostline, where they are pushed by the sun in the early development of the system. Most elements but iron were pushed out of the innermost system, rocky dust remained in the inner system, water was pushed back to 2.7 AU, helping with the formation of Jupiter and so on. If you find out as what molecule chlorine can be found in the proto planetary disc and there is indeed a dominant chlorine carrier, a planet formed at this molecules Frostline should be rich in chlorine. $\endgroup$
    – TheDyingOfLight
    Commented Jun 1, 2020 at 6:00
  • $\begingroup$ +1 because I like the question. Note that for all we know about the formation of our own planet, in reality we're just scratching the surface of solar system formation. We're obviously limited by not having a lot of other systems to compare ours to - and we do get regularly surprised by new astronomical finds (that planet's doing what?). So I ask... is it necessary for this to happen at the planet formation stage? Or could a talented botanist describe how it could have come to pass via flora? $\endgroup$
    – JBH
    Commented Jun 1, 2020 at 6:09
  • $\begingroup$ Also like the question. There seem to be an excess of perchlorates on the Martian surface, which I suppose could be related. It seems your question and the amount of perchlorate could be related to the amount of water on Mars over time. Perhaps that is related to the frost-line and volatility too. $\endgroup$
    – UVphoton
    Commented Jun 1, 2020 at 12:53

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The probable reason why more chlorine is seen on Mars compared to Earth is concentration by loss of water. Many chlorides are soluble in water and would accumulate in an ocean due to weathering. Mars did have an ocean billions of years ago and that ocean was probably salty like ours. But as Martian gravity is relatively weak water has been lost over millions of years into space by photo dissociation of water into hydrogen and oxygen in the upper atmosphere and other processes. This has concentrated the chlorine into a salty crust where other chemical reactions with oxygen have generated perchlorates.

So a small planet at an appropriate distance from its star might accumulate a huge ocean during formation. That water might well dissolve chlorides out of the crust by weathering over millions of years. If the planet was also hit by many large objects, (even more than hit Earth) it might be subject to a lot of mixing and disruption of the crust due to those impacts allowing water to leach soluble minerals out of a vast quantity of crustal and impact fragments. If that ocean was then stripped away as per Mars the result could be highly enriched quantities of elements like chlorine on the surface in ionic form.

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